The invention relates to a new complex of an element of transition group IV or V for forming an improved precursor and/or precursor combination for use in chemical vapor deposition (CVD). The term xe2x80x9cprecursor combinationxe2x80x9d refers to a mixture (e.g. a metal complex or precursor and a solvent) which is vaporized at commencement of a CVD reaction.
Precursor combinations are known from U.S. Pat. No. 5,820,664. In order to effect chemical vapor deposition of, for example, SrBi2Ta2O3 (SBT), use is made of a precursor combination including [Ta(OiPr)4thd] (see column 6, line 42), in which a Ta complex with an alkoxide ligand having an xcex1 proton and thus a potential reducing agent (to form the corresponding ketone) is present.
The precursor Bi(Ph)3 has been replaced by Bi(thd)3 in order to obtain stable Bi2O3 deposition rates (see a presentation by F. Hintermaier et al., given at the International Symposium on Integrated Ferroelectrics, Monterey, Calif., USA, 1998). However, Bi(thd)3 is easily reduced to the metal in the presence of a reducing agent. Thus, for example, hydrolysis of the precursor [Ta(OiPr)4thd] liberates HOiPr which acts as a reducing agent or in a ligand exchange reaction according to the equation:
Ta(OiPr)4thd+Bi(tdh)3xe2x88x92 greater than Ta(thd)2(OiPr)3+Bi(thd)2(OiPr)
forms (Bi(thd)2(OiPr) which undergoes an internal redox reaction with reduction of the Bi3+ metal ion to liberate elemental bismuth.
It is accordingly an object of the invention to provide a new complex, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known complexes of this general type, which is suitable as a precursor and which remains stable in chemical vapor deposition using a Bi3+ precursor and/or, in the case of a ligand exchange reaction, promotes formation of a by-product which is thermally stable under reaction conditions of a CVD technique and in which bismuth continues to be present in oxidized form.
This object is achieved according to the invention by a complex which has only alkoxides without an xcex1 proton and very bulky ligands as complex formers.
The invention provides a complex of the formula:
M (L)x(R3Cxe2x80x94Oxe2x80x94)y-x
where:
M is a stable central atom from transition group IV or V of the Periodic Table;
L is a xcex2-diketonate, a xcex2-ketiminate and/or a xcex2-diiminate;
(R3Cxe2x80x94Oxe2x80x94) is an alkoxide ligand in which R may be identical or different and are each an alkyl radical which has from 1 to 24 carbon atoms and may be branched or linear and substituted or unsubstituted and/or complexed;
x is not equal to zero and is from 1 to 4; and
y is not equal to zero and is 2, 3, 4 or 5, depending on the oxidation state of the central atom.
The invention also provides for the use of this complex for the deposition of ferroelectric, paraelectric and high-xcex5 layers.
Finally, the invention provides a precursor combination including the new complex as a precursor.
Preference is given to using tantalum or niobium as a central atom of the complex.
Preference is given to using the tert-butyl radical and/or the tert-pentyl radical as a tertiary radical on the alkoxide ligand.
In an advantageous embodiment, thd, namely 2,2,6,6-tetramethyl-3,5-heptanedionate, is used as a bulky ligand.
In a further advantageous embodiment, 4 alkoxide ligands and one (thd) ligand are bound to the central atom of the complex.
The term xe2x80x9cstable central atom of the complexxe2x80x9d refers to an element of the Periodic Table having a most abundant isotope which does not undergo radioactive decay. Preference is given to a metal selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta. Particular preference is given to using Ta.
The term xe2x80x9calkoxide ligandxe2x80x9d refers to the alcoholate ligand which is bound by the oxygen atom of the alcohol group and has the formula:
xe2x80x94Oxe2x80x94CR3
where R may be identical or different and are each an alkyl radical which has from 1 to 24 carbon atoms and may be branched or linear and substituted or unsubstituted and/or complexed. It is advantageous, for example for the alcoholate ligand to contain ether, amine and/or sulfide groups which can additionally act as donors toward the central atom.
Particular preference is given to using an alkoxide ligand of the formula:
xe2x80x94Oxe2x80x94C (CH3)3-n[(CH2)mxe2x80x94CH3]n
where:
n is in a range from 0-3; and
m is in a range from 0-3.
Preference is given to using derivatives of a xcex2-diketonate, a xcex2-ketiminate and a xcex2-diiminate.
2,2,6,6-tetramethyl3,5-heptanedionate is preferably used. Other preferred ligands are, for example, acetylacetonate (acac); hexafluoropentanedionate (hfac); 1,1,1-trifluoro-2,4-pentanedionate (tfac); 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate; 2,2,7-trimethyl-3,5-octanedionate; 1,1,1,5,5,6,6,7,7,7-decafluoro-2,4-heptanedionate; and finally 1,1,1-trifluoro-6-methyl-2,4-dionate.